A diesel particulate filter (DPF hereafter) is known as a device for removing PM discharged from a diesel engine. A DPF is a filter which traps particulate matter (PM) contained in exhaust gas, and is required to perform so-called regeneration processing to remove the trapped PM through oxidation when the trapped PM amount reaches a fixed amount. A method of heating the DPF using a heating device such as a heater when the trapped PM amount reaches a predetermined value is known as a method of performing this regeneration processing, but with this method, a regeneration device must be provided, leading to an increase in cost.
In a known method for solving this problem, an oxidation catalyst (DOC hereafter) is provided in an exhaust passage on the upstream side of the DPF for oxidizing NO contained in the exhaust gas to generate NO2. The DPF is then regenerated by oxidizing the PM trapped the filter continuously using the heat of the exhaust gas and the NO2. In an exhaust gas purification device disclosed in JP2001-280118A, regeneration is performed using this method. The DOC and DPF are housed in a single case, and when the PM amount trapped in the DPF exceeds a predetermined amount, the exhaust gas temperature is raised through post-injection of the fuel and so on in order to burn the trapped PM.